Climate-based Daylight Modelling
Predicting annual daylight performance
Climate-Based Daylight Modelling (CBDM) uses local weather data to evaluate how daylight is likely to perform within a building over the course of a typical year.
Unlike traditional daylight calculations, which assess a single overcast sky condition, CBDM provides a far more realistic understanding of how natural light interacts with architecture under changing weather, seasons and times of day.
This enables architects and design teams to make better-informed decisions from the earliest design stages, helping create buildings that maximise natural daylight, improve occupant comfort and wellbeing, reduce energy consumption, and demonstrate compliance with standards such as the Department for Education daylight requirements and BS EN 17037.
At Lighting Analysts, Climate-Based Daylight Modelling forms the foundation of our daylight consultancy, allowing us to optimise buildings not simply for compliance, but for the people who use them.
CBDM Metrics
Daylight Autonomy (DA) is the percentage of time that a target illuminance level is achieved, or exceeded. The measure is typically taken across a year, during hours of operation. Theoretically, DA could be measured daily, weekly, monthly or any other time period. DA is typically measured across the working plane of a space, with DA values being calculated at each grid point.
Spatial Daylight Autonomy (sDA) defines the percentage of time that a target illuminance (typically 300lux) is achieved across a defined percentage of floor area (typically 50%).
In England, the DfE requires and sDA (300/50%) ≥50% across at least 50% of the working plane, during occupied hours.
Useful Daylight Illuminance (UDI) is a measure of daylight availability, during hours of operation. Rather than a specific target illuminance, UDI analyses the percentage of time that a target range of illuminances can be achieved.
Typically, UDI is divided into three ranges; UDI-s (<100lux), UDI-a (100-3000lux), and UDI-e (>3000lux).
UDI-s (<100lux) is a measure of daylight availability, during hours of operation, below the target range of 100-3000lux.
In England, the DfE GDB specifies that the UDI-a (100-3000lux) should be ≥80% (ave), across the working plane of each space, during occupied hours. This means that the UDI-s & UDI-e cannot exceed 20% (combined).
UDI-a (100-3000lux) is a measure of daylight availability, during hours of operation, within the target range of 100 - 3000lux.
In England, the DfE GDB specify that the UDI-a (100-3000lux) should be ≥80% (ave) across the working plane of each space, during occupied hours.
UDI-e (>3000lux) is a measure of daylight availability, during hours of operation, above the target range of 100-3000lux.
In England, the DfE GDB specify that the UDI-a (100-3000lux) should be ≥80% (ave) across the working plane of each space, during occupied hours. This means that the UDI-s + UDI-e cannot exceed 20% (combined).
